The multi-core optical fiber can transmit mass information, since each of the plurality of cores functions as an optically independent optical waveguide. Generally, in the cross-section perpendicular to the fiber-axis of the multi-core optical fiber, two or more cores are arranged in symmetry (rotational symmetry or line symmetry) (Non-patent Literature 1). Symmetrically arranging the plurality of cores makes it possible to arrange the cores with high density in the section of the multi-core optical fiber and to control the cross talk between cores.
FIG. 1 is a cross-section of a multi-core optical fiber 2 for a comparative example. In the multi-core optical fiber 2, seven cores 10 to 16 which extend along the fiber-axis direction are surrounded by a shared cladding 30. The core 10 is arranged at the center of the section of the optical fiber 2, and the six cores 11 to 16 are arranged at equal intervals on the circumference of a circle such that the core 10 is located at the center of the circle. That is, the seven cores 10 to 16 are arranged in a manner having 6-fold rotational symmetry.
In a case where two or more cores are symmetrically disposed in the section, the cores having symmetrical relations cannot be identified. In the multi-core optical fiber 2, the respective six cores 11 to 16 cannot be identified. Even if the six cores 11 to 16 differ in terms of the core diameter or the refractive index, it would be difficult to identify each of the six cores 11 to 16.